pH of 33% KOH?

[passedpawn]

I made a 33% solution of KOH and measured it and I got about exactly 16 pH. My meter won't read above 14, so I diluted it 1000x and measured 13. So, I conclude 16 pH (pH scale is logarithmic).

But I did a back of the envelope calculation and concluded that the pH should be 14.77 pH. My math: Mole of KOH = 56g, 33% solution is 330g of KOH. in 670g of water. 330g / 56g = 5.89 mol of KOH.

pH = 14 + log(5.89) = 14.77 pH (henderson-hasselbalch)

 

[DeeAnna]

First, your formula is incorrect --
pH = 14 - log[OH-]

Second, the pH ranges from 0 to 14 by definition. There is no such thing as a pH of 16.

Third, you're making a huge assumption that a single data point at 0.033% can be accurately extrapolated to 33% concentration. You need to run a titration curve to get any kind of decent accuracy out of experimental data. It makes no sense to measure one data point and assume you can just run the math from there.

 

[passedpawn]

My question is, what SHOULD the pH of 33% KOH solution be? I assumed this would be easy to find on the webz but was wrong. So I attempted to measure myself, with solid KOH. I think the dilution math here is correct in order to dilute my sample. Are you suggesting my measurement errors would be significant and result in erroneous conclusion? That's very likely. My scale weighs to 0.1g resolution.

pH + pOH = 14. So, pH = 14 - pOH. Now, the "p" is actually a convention for -log(), which is why pH = 14 + log(OH). pH can be greater than 14. No weirdness here... 14 is just a convention that has been dragged from early chemists. No doubt that most meters, like mine, only work to 14.

 

[DeeAnna]

If you refuse to accept basic tenets of chemistry and of basic chemical laboratory practice, there's no point to me trying to help you. I'm outta here.

 

[passedpawn]

I really didn't want to get into a chemistry discussion, I just want the answer to pH of 33% kOH.

But I don't mind the discussion, either. 0 and 14 are not limits to pH... it can go above and below. I still think 14.77 is correct (based on 33% or 5.89M of KOH), and I even think it could go higher.

For example, I think the max pH is 15.74. There are 55 mol in 1L of water (H2O = 18g, 1L of H20 = 1000g, 1000g/18g = 55.5M). So assuming KOH dissociates completely in water, KOH+H2O->KO+H3O, we can assume a OH- concentration of 55.5M, and 14+log(55.5) = 15.74 pH.

 

[DeeAnna]

Okay, I stand corrected -- the pH can theoretically go above 14 and below zero. But I stand behind my third point about correct lab procedures -- you can't use only one data point, extrapolate that far, and expect to get accurate answers, especially using the equipment you say you're using.

"...My question is, what SHOULD the pH of 33% KOH solution be?..."

Frankly I don't have an answer for you other than to do what you've already done -- calculate a theoretical value. Are you not satisfied with your math?

Why do you want this information? If you provided some background, you might get more and better answers.

 

[passedpawn]

Yes, I'm satisfied with my math. I'm making lye from oak ash. Thanks for the reply.

[edit] So I am boiling to increase the pH. I only have pH measurements to get to my necessary 33% concentration, which my math tells me is 14.77pH, which I can't measure with my meter, which is why I need to dilute it. So it's not that complicated, but I was checking on the number, but I think I'm satisfied it's correct now.

 

[kayak1987]

Yes, I'm satisfied with my math. I'm making lye from oak ash. Thanks for the reply.

[edit] So I am boiling to increase the pH. I only have pH measurements to get to my necessary 33% concentration, which my math tells me is 14.77pH, which I can't measure with my meter, which is why I need to dilute it. So it's not that complicated, but I was checking on the number, but I think I'm satisfied it's correct now.

There are too many wrong assumptions, mistakes and little knowledge of chemistry in what you write,
so that you will not get into any significative result in this way.
The only way to make something as near as possible to what you want, is to follow a known procedure like "boil X g of ash in Y g of Water.

Trust me this is much much more complicated than what you think it is

 

[DeeAnna]

Yes, I'm satisfied with my math. I'm making lye from oak ash....

Oh. That's a critically important tidbit.

Your math skills might be fine, but you've not done your chemistry homework yet.

Apparently you don't know wood-ash lye is impure potassium carbonate. It is absolutely and most definitely not potassium hydroxide. There might be a skosh of sodium carbonate in there, but in the end it's all a carbonate mixture. You can boil to dryness and you still won't have what you think you have.

edit: And you can accurately measure pH concentration in an unknown solution by titrating with a known acid or base solution to the desired pH endpoint and then back calculating.

 

[passedpawn]

Oh. That's a critically important tidbit.

Your math skills might be fine, but you've not done your chemistry homework yet.

Apparently you don't know wood-ash lye is impure potassium carbonate. It is absolutely and most definitely not potassium hydroxide. There might be a skosh of sodium carbonate in there, but in the end it's all a carbonate mixture. You can boil to dryness and you still won't have what you think you have.

edit: And you can accurately measure pH concentration in an unknown solution by titrating with a known acid or base solution to the desired pH endpoint and then back calculating.

Thanks. I've got some muriatic acid. I think I can use that. Doing the titration this weekend to measure total alkalinity. Once I can get that number, I can move forward making some use (or not!) with my ash water.

Also, a new electrode for my pH meter arrived today. Previous, coincidentally, was spent. New cal solutions too. Will use that to look for inflection point in titration curve.

 

[kayak1987]

I don't want to destroy your curiosity, keep doing that experiments, but you need to know that you are doing this just for fun!

This because what you are doing is a "chemistry puzzle" that will be quite hard to solve also for a good chemist with a full lab.

Consider you are not in front of a simple solution but a quite complex one, made of many different compounds, in unknown percentages.
The acid you are going to use is not an analyte.
These two things makes your titration useless.
This because of many problems,
You won't have a pH definite drop because of the mixture of strong and weak bases, like carbonates an hydroxides.
And even if you can see a drop you can't get any information on how much KOH you have in the solution. Or in the worst case it will lead to a bad interpretation of the results you get.

So do your experiments but with the consciousness that they won't give you the info you are searching!

P.S. if if you want to make a titration you can't use HCl but oxalic acid or acid potassium phtalate, or, titrate a diluted solution of HCl with a standard like sodium carbonate and then use it immediatly.

(Anyway, as said, the bigger problem is not the titrant but the mixture you are going to analyze)

 

[Zany_in_CO]

200g KOH
600g Water
800g 25% KOH Solution. <<<pH over 14 (BIG Time!)

Added 25 grams total White Vinegar <<<pH didn't budge to 14. Not even close.

Question: How do I lower the pH of 25% KOH solution to neutral pH 7?
Is it possible to lower the pH by adding enough water to reach KOH 3% solution?
If so, how do I do that?

Chemistry/Math Challenged.

 

[Zany_in_CO]

YOO HOO! Anybody???

 

[BodyWasher]

Screw all this theory. Buy a pH meter, find out, and let us know